This invention relates to a low noise television system having means for suppressing increase of noise as maintaining improved picture quality in television broadcast transmission and reception.
U.S. Pat. No. 4,779,133 discloses a television system of amplitude modulation (AM) type which uses a technique as follows for the purpose of utilizing a transmission bandwidth of a luminance signal to the greatest extent and suppressing noise in a video signal produced in a receiver. In the transmission side, a frequency characteristic of the luminance signal to be transmitted is controlled so that it varies continuously between a flat frequency characteristic and a predetermined preemphasis frequency characteristic depending upon a low frequency component in the luminance signal. In the reception side, the received luminance signal is controlled with a frequency characteristic which varies continuously between the flat frequency characteristic and a deemphasis frequency characteristic which is opposite to the abovementioned preemphasis one depending upon the low frequency component in the received luminance signal, to obtain a luminance signal for image display.
This prior art technique has enabled to transmit a signal having a preemphasis effect even in the AM television system. More particularly, a maximum preemphasis is applied in the transmission side to a portion of the luminance signal including a little low frequency component, namely, exhibiting a small amplitude, while almost no preemphasis is applied to a portion of the signal including a big low frequency component, namely, exhibiting a large amplitude. This results in a preemphasis effect as preventing occurrence of overmodulation. Moreover, a signal treated by using this technique has a compatibility to a classic receiver having no function regarding deemphasis and it generally raises definition of a displayed image and improves quality of the image in the receiver.
Although, when the abovementioned signal treatment is effected in the transmission side, a desired effect is obtained in most of the image, it may result in overshoot and undershoot having some extent of magnitude and a waveform determined in accordance with a preemphasis constant, as shown in FIG. 6, in a portion in the displayed image in which an amount of change of the level of waveform (hereinunder referred to as "transient") of the luminance signal is large. Such overshoot and undershoot are originally unwanted. The treated luminance signal waveform in this portion has a chance for resulting in instantaneous overmodulation of the receiver exceeding a desired upper threshold of signal level, though it is a matter of course that the excessive amount of the signal level is very little as compared with the case where only a simple preemphasis is effected without use of the abovementioned technique. In addition to the excess of the signal waveform level, a contour of the displayed image in this portion may be excessively emphasized to appear strange.